Admittance spectroscopy in kesterite solar cells: Defect signal or circuit response

Unlike Cu(In,Ga)Se2 based solar cells, Cu2ZnSn(S,Se)4 solar cells show a strong increase in series resistance with decreasing temperature. In this study we deduce the series resistance from temperature dependent current-voltage measurements on a 5.5% efficient Cu2ZnSnSe4 solar cell. By applying a simple circuit model an increasing series resistance with decreasing temperature alone results in a capacitance step within the C-f profile. We show that this step needs to be distinguished from a step caused by a defect state or a carrier freeze-out. Consequently, the deduced activation energy is strongly distorted by the circuit response.